The invention generally relates to methods and apparatus for determining wavelength of radiated light such as, for example, in spectroscopic analysis. More specifically, the invention relates to methods and apparatus for characterizing and utilizing light responsive devices including devices that have temperature-sensitive spectral responses.
Spectroscopic analysis is used in many different types of operational systems and analytical apparatus. The broadband frequency or wavelength characteristic of visible and invisible electromagnetic energy is particularly useful in sensors and analyzers subject to harsh environmental conditions and electromagnetic interference. For example, it is widely anticipated by practitioners in the aerospace industry that optics-based sensors will be widely used in next generation of military and commercial aircraft.
One of the more common techniques used for analyzing the wavelength characteristic of electromagnetic radiation is diffraction. Diffraction analysis is often feasible because many of the various components used are mechanical and thus less sensitive to temperature variations. However, such apparatus that use diffraction tend to be complex and require expensive components and critical alignments in order to provide accurate wavelength detection over a broad spectrum.
A known device useful for determining wavelength of electromagnetic radiation is commonly referred to as a color sensor. A color sensor is typically a semiconductor photoelectric device that has electrical parameters or characteristics responsive to electromagnetic energy incident on the device. The use of a color sensor has significant advantages over diffraction and other wavelength analyzers due to the sensor's simplicity, low cost and small size.
However, a significant disadvantage of semiconductor color sensors that has prevented their use in precision transducers and control devices is that color sensors are highly sensitive to the operating temperature of the device. If the ambient operating temperature of the device can be known and controlled, then a predictable relationship exists between the color sensor's output and the wavelength of incident light on the device. However, as the ambient temperature changes the entire spectral responsive curve of the color sensor also changes. For example, at 20.degree. C. a color sensor may provide an output that corresponds to a wavelength of 835 nanometers, but the same incident light will cause an apparent wavelength reading of perhaps 845 nanometers at 30.degree. C.
Such temperature variations are not particularly critical in applications where there is a wide tolerance for detected values. For example, if the color sensor is simply being used as a color detector where the peak wavelengths are several hundred nanometers apart, the temperature-induced shifts can easily be compensated or ignored. However, applications such as aircraft flight controls and optical sensors typically impose a much tighter requirement on detecting wavelength. Thus, an optical transducer used as part of a flight surface control may require a wavelength analyzer that can discriminate wavelength variations of only fractions of one nanometer or less. At such tight tolerances, the temperature-induced variations in a semiconductor color sensor become significant and in fact may far exceed the allowed detector tolerance. Therefore, to date, such color sensors have not been utilized in precision transducers and optical sensors subject to hostile environmental conditions, such as extreme temperature variations typically encountered in aircraft and space environments.